Download Laboratory measurements of the seismic velocities and other

Geophysical Journal International
Geophys. J. Int. (2011) 184, 405–415
doi: 10.1111/j.1365-246X.2010.04845.x
Laboratory measurements of the seismic velocities and other
petrophysical properties of the Outokumpu deep drill core samples,
eastern Finland
Tiiu Elbra,1 Ronnie Karlqvist,2 Ilkka Lassila,2 Edward Hæggström2
and Lauri J. Pesonen1
1 Department
2 Department
of Physics, Division of Geophysics and Astronomy, University of Helsinki, Finland. E-mail: [email protected]
of Physics, Division of Materials Physics, University of Helsinki, Finland
SUMMARY
Petrophysical, in particular seismic velocity, measurements of the Outokumpu deep drill
core (depth 2.5 km) have been carried out to characterize the geophysical nature of the
Paleoproterozoic crustal section of eastern Finland and to find lithological and geophysical
interpretations to the distinct crustal reflectors as observed in seismic surveys. The results
show that different lithological units can be identified based on the petrophysical data. The
density of the samples remained nearly constant throughout the drilled section. Only diopsidetremolite skarns and black schists exhibit higher densities. The samples are dominated by
the paramagnetic behaviour with occasional ferromagnetic signature caused by serpentinitic
rocks. Large variations in seismic velocities, both at ambient pressure and under in situ
crustal conditions are observed. The porosity of the samples, which is extremely low, is either
intrinsic by nature or caused by decompaction related to fracturing during the core retrieval.
It is noteworthy that these microfractures have dramatically lowered the VP and VS values.
From the measured velocities and density data we have calculated the seismic impedances,
Young’s modulus and Poisson’s ratios for the lithological units of the Outokumpu section and
from these data the reflection coefficients for the major lithological boundaries, evident in
the surveyed section, were determined. The data show that the strong and distinct reflections
visible in wide-angle seismic surveys are caused by interfaces between diopside-tremolite
skarn and either serpentinites, mica schist or black schist.
Keywords: Body waves; Wave propagation; Acoustic properties; Crustal structure.
1 I N T RO D U C T I O N
Our knowledge of the structure of a typical Precambrian continental crust is important when one tries to understand the geological
processes and evolutionary history of the shield areas. Over the
past decades worldwide seismic surveys have improved our knowledge of the crustal structures (e.g. Christensen & Mooney 1995;
Kukkonen et al. 2006; Brown et al. 2009; Kern et al. 2009). Even
though numerous wide-angle reflection seismic transects covering
the entire Fennoscandian Shield have been carried out (Kukkonen
et al. 2006; Heikkinen et al. 2007; Kukkonen et al. 2007a;
Schijns et al. 2007), detailed information about the distribution
of rock types within the crust, as well as information about their
petrophysical properties is still needed. Several attempts to build
lithological models of the crust and upper mantle of the Fennoscandian Shield have been carried out (e.g. Korja et al. 2006; Kuusisto
et al. 2006; Sorjonen-Ward 2006) based on surface geology, geophysical data and elastic velocity models. However, these models
C
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Geophysical Journal International have two drawbacks. First, the knowledge of the various lithological
units constituting the continental crust is poor and secondly, only a
few elastic property determinations are available for the rock units
exposed to real crustal pressures and temperatures.
In recent years, scientific drilling has become a successful
tool for providing detailed petrophysical and lithological data
necessary to understand the structure, composition and physical
state of the crust (e.g. Kola super deep hole: Kern et al. 2001;
Chesapeake Bay impact structure: Elbra et al. 2009; Powars et al.
2009; Chicxulub impact structure: Vermeesch & Morgan 2004).
For this reason, the Geological Survey of Finland (GSF) carried out
the Outokumpu Deep Drilling Project in 2004–2005, in cooperation with the International Continental Scientific Drilling Program
(ICDP). The project was carried out to understand the deep structure
of the ore province in the Precambrian terrain of eastern Finland
where the strong seismic reflectors are visible in numerous seismic
reflection surveys (Heikkinen et al. 2007; Kukkonen et al. 2007a).
The focus of the drilling effort (Kukkonen et al. 2007a) was to
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Accepted 2010 October 8. Received 2010 October 6; in original form 2010 January 29
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Tiiu Elbra et al.
penetrate the observed strong seismic reflectors within the upper
crust. The Outokumpu (hereafter called OKU) deep drill core offers reference material to investigate the physical properties of a
Paleoproterozoic crustal section as a function of depth. This core
provides an opportunity to distinguish the role of burial or tectonic effects on the physical properties of the rocks. Such information could also benefit the future exploring surveys of the OKUtype mineralizations in the area by providing improved knowledge
about the physical properties of the upper crustal rocks. Furthermore, the deep biosphere and the origin of the saline fluids and
gases are of interest (Kukkonen et al. 2007a) and, thus, rock physical property data, notably porosity and microcracking, can provide important constraints in these studies. We emphasize however that laboratory measurements of small rock samples represent only a small part of the crustal section. Hence they don’t
necessarily fully represent the same in situ unit detected by seismic surveys or even by geophysical loggings (e.g. VSP surveys;
Schijns et al. 2007) for two reasons. First, the deep seismic soundings and the borehole loggings integrate much wider areas of the
crust in their signals. Secondly, the surveys and loggings do not
suffer fracturing caused by decompaction during core retrieval (e.g.
Kern et al. 2001).
In this paper, we present petrophysical, especially ultrasonic seismic velocity, density and porosity data of the Outokumpu deep drill
core. From the petrophysical data we obtain the seismic impedances,
Young’s modulus, Poisson’s ratios as well as the reflection coefficients for various lithological units and their boundaries. These data
are used to find explanations to the distinct crustal reflectors seen in
seismic reflection studies, especially the strong reflectors at depths
of ∼1300–1500 m.
2 R E G I O NA L G E O LO G Y A N D
GEOPHYSICS
The Paleoproterozoic Outokumpu region is located in the
Fennoscandian Shield, in eastern Finland, close to the ArcheanProterozoic boundary zone (Fig. 1a). It is known for its occurrences
of early Proterozoic Cu–Zn–Co sulphide ore deposits (Koistinen
1981; Loukola-Ruskeeniemi 1999 and references therein). The
Outokumpu ores are associated with a lithologic complex called the
Outokumpu Association that consists of serpentinites, dolomites,
calc-silicate rocks, siliceous rocks and metamorphosed black shales
(Kukkonen & Šafanda 1996; Airo & Loukola-Ruskeeniemi 2004).
This assemblance, also described as an ophiolitic complex, meanders along as a discontinuous band for more than 200 km (Papunen
1987; Kukkonen & Šafanda 1996). The rocks of the Outokumpu
ophiolitic complex are embedded in mica schists and -gneisses
which are the main rock types in the area (Kukkonen & Šafanda
1996). The Outokumpu basin has been extensively studied by the
Outokumpu Company (numerous drillings) and the GSF, including
geological, geochemical and geophysical mappings. The geophysical surveys show generally positive magnetic anomalies, which
correlate with rock horizons associated with ore mineralization
(Ruotoistenmäki & Tervo 2006). On the other hand, the distinct
Figure 1. (a) Regional geology of Finland, showing the distribution of Archean rocks in Fennoscandian Shield (modified after Sorjonen-Ward & Luukkonen
2005) and the location of the FIRE transects, including OKU-1 line (marked in yellow) which follows the same road as one of the FIRE-3 sections (after
Kukkonen et al. 2006). (b) Lithology of OKU deep drill core (drawn after Västi 2005).
C 2010 The Authors, GJI, 184, 405–415
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Geophysical Journal International Laboratory seismic data of the Outokumpu core
gravity anomalies demonstrate that complex block tectonics has
taken place, reflecting either basement doming (e.g. Sotkuma) or
granitic batholiths (Maarianvaara; Ruotoistenmäki & Tervo 2006).
Both magnetic and gravity data reveal also fracture or shear zones.
Furthermore, the central Fennoscandia is covered with numerous
wide-angle reflection seismic surveys and seismic array studies
(Kukkonen et al. 2008), for example, the FIRE (Finnish Reflection Experiment) project which includes profiles transecting the
Outokumpu area (Fig. 1a) and reveal distinct crustal reflectors
(Kukkonen et al. 2006). Despite the numerous investigations the
genesis, tectonic evolution and causes for the seismic reflectors
of the Outokumpu basin are still incompletely understood as the
depositional basement is not met and since the assemblance itself is severely altered and tectonized (Park 1988). A review of
the geology, geophysics and tectonic evolution of the Outokumpu
area is published by Sorjonen-Ward (1997); Tyni et al. (1997),
Airo & Loukola-Ruskeeniemi (2004) and Ruotoistenmäki &
Tervo (2006).
3 OUTOKUMPU DEEP DRILLING
The Outokumpu drilling was carried out in 2004–2005 by the contractor NERDA–a Russian governmental company. The drill site
is located 2 km SE from the Outokumpu town (Kukkonen et al.
2009; Fig. 1a) featuring a borehole that penetrates through metasediments and ophiolitic rocks into pegmatite-granite at its deepest part
(Fig. 1b). The borehole is 2516 m deep and the full sequence was
cored with a core recovery of 80 per cent. Although the hole was
initially vertical it soon deviated by a few degrees (up to 250 m
on surface projection) towards NW and W (Kukkonen et al.
2007a; 2009). The geophysical down-hole logs and hydraulic experiments were performed throughout the hole (Kukkonen et al.
2007a; Schijns et al. 2007). The OKU borehole has been kept
open as a ‘Deep Geolaboratory’ for in situ experiments and
monitoring.
Lithologically the topmost part of the core (33–1314 m; hereafter
called upper schist series; Fig. 1b) consists of fine-grained strongly
foliated mica schists containing some biotite gneiss, chlorite-sericite
schist and black schist interlayers. This mineralogically most uniform area of the core (Kukkonen et al. 2007a), is made up of biotite,
quartz and plagioclase minerals, exhibiting strong preferred orientation of elongated platy mica grains nearly perpendicular to the
core axis (i.e. z-axis; Kern et al. 2009). The upper schist series
is followed by the ophiolitic complex (1314–1515 m), consisting
of serpentinites, weakly foliated diopside and tremolite skarns and
black schists (Kukkonen et al. 2007a; Kern et al. 2009). Mineralogically the serpentinites consist of fine-grained platy antigorite,
fibrous chrysotile and randomly distributed oxide minerals (e.g.
magnetite). The diopside-skarns of the ophiolitic complex are made
up of coarse-grained hornblende and clinopyroxene which are irregularly distributed in a fine-grained hornblende, diopside matrix
(Kern et al. 2009). Pegmatite granites (2013–2516 m; pegmatoid
series) in the lowermost part of the core underlie 500 m thick mica
schist layer (1515–2013 m; lower schist series). The pegmatoids
are dominated by randomly distributed coarse-grained quartz and
feldspar minerals. A weak shape-preferred orientation of the minor
minerals muscovite and biotite was observed only at macroscopic
scale (Kern et al. 2009) with nearly horizontal foliation. The more
extensive grain size distribution as well as the chemical- and modal
composition of the samples from the OKU deep drill core is published by Kern et al. (2009).
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Geophysical Journal International 407
4 M E T H O D S A N D I N S T RU M E N T S
Samples used in the current study come from the OKU deep drill
core. Cylindrical samples of two different diameters (ø ∼ 4 cm,
L ∼3–7 cm and ø ∼ 2.5 cm, L ∼2–2.5 cm) were used. The sample
sets were prepared at the GSF.
Basic petrophysical parameters, such as wet density (ρ), porosity
(φ), magnetic susceptibility (κ) and intensity of the natural remanent magnetization (NRM) were measured for the selected samples
(>350; mostly samples with ø ∼ 2.5 cm) to characterize the physical properties of the drilled section. The density and porosity were
determined using the Archimedean principle based on weighting
water saturated and oven dried samples in air and in water. For magnetic susceptibility RISTO-5 (1025 Hz operating frequency and
48 Am−1 field intensity) and the AGICO KLY3S (875 Hz operating frequency and 300 Am−1 field intensity) kappabridges
were used. The NRM measurements were performed using a
Superconducting Rock Magnetometer (Model 755, 2G Enterprises)
in combination with an AF demagnetizer (Model 2G600, Applied
Physics) on the ‘paleomagnetic’ samples (ø ∼ 2.5 cm). The seismic P-wave velocities (longitudinal; VP0 ) in more than 1700 watersaturated samples were measured (in ≤1 m intervals whenever the
core allowed) at ambient pressure with an apparatus (93 Hz frequency) developed at the GSF. The velocity was calculated using
the sample length and the P-wave travel time through the cylindrical
sample. The travel time was determined using two identical transducers as transmitter and receiver, respectively (Airo et al. 2007).
All VP0 measurements were conducted in water to provide good
contact between the transducers and rocks.
To better understand the cause of the seismic reflectors observed
in seismic reflection surveys and in acoustic soundings (e.g. FIRE;
Kukkonen et al. 2006) we built an ultrasonic instrument (Fig. 2)
to measure the seismic P- and S-wave velocities (VP and VS , respectively) in the drill core samples under crustal pressure and temperature (Lassila et al. 2010). The system comprises two custombuilt transducers with longitudinal (1.0 MHz frequency) and shear
(1.1 MHz frequency) Pz27 elements, a hydraulic cylinder (HSS 254,
Hi-Force) and a load cell (Model 53, Honeywell) residing inside a
metal cradle. The uniaxial compressional loading is produced with a
hydraulic cylinder that is driven with a computer controlled, 3-way,
load holding pump (HEP 2142S3, Hi-Force). The induced load is
monitored with a load cell. The transducers are excited simultaneously (200 V, negative spike) with a pulser (5058PR, Panametrics)
and the signals that have passed through the sample are sampled
with an oscilloscope (9410, LeCroy). The maximum load provided
by the system is limited by the load cell to 25 tons (480 MPa, which
corresponds to a crustal depth of about 18 km, for a cylinder with
ø ∼ 2.5 cm diameter). The change in sample height is measured
with 1 μm resolution during the compression with a digimatic indicator (543–250B, Mitutoyo). All the measured ultrasonic signals,
the sample temperature and height, as well as the applied load are
recorded. The calibration signals are subtracted from the actual measurements to cancel out effects of transducer housing compression
and heat expansion (Karlqvist 2009; Lassila et al. 2010). A brass
holder supporting the sample from the side was used to avoid sample cracking under uniaxial pressure; the close-fitting limited also
the sample expansion. The supporting force produces radial stress,
whose magnitude depends on the Poisson’s ratio of the sample. This
force decreases the stress difference between that occurring along
the axial and the radial directions. The ultrasonic velocities were
determined from the measured time of flight data and the sample
height.
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Tiiu Elbra et al.
Figure 2. The measurement assembly. The sample (1) resides inside the
sample holder (2) between the transducers (3). The distance gage (4) is
attached to the base of the upper transducer while the counterpart for it is
attached to the top of the lower transducer. The load cell (5) that holds the
upper transducer is fixed to the piston of the hydraulic cylinder (6) that is
supported from the top part of the steel frame (7). (See details in Lassila
et al. 2010).
We used over 100 water-saturated samples to simulate the upper
crustal conditions. The pressure employed in the VP and VS measurements ranged from 3 to 40 MPa depending on actual sample
depth (125–1500 m). Due to the paleoclimatic ground surface temperature history the subsurface temperature in Outokumpu is low
(Kukkonen & Šafanda 1996)–at the final 2.5 km depth only 40 ◦ C
(Kukkonen et al. 2007b). Tests showed no significant temperature
dependence of the velocity within the interval 20–40 ◦ C; hence
the VP and VS measurements were conducted in room temperature
altering only the load pressure. All velocity measurements were conducted in the direction normal to the foliation plane (z-axis), that is,
normal to preferred orientation as described previously. The VP and
VS provide estimates of (i) seismic impedances (ZP ), VP /VS ratios,
Poisson’s ratios (ν) and Young’s modulus (E) for the lithological
units of the drilled section and also, (ii) the reflection coefficients
(RC ) of the lithological boundaries.
5 R E S U LT S
5.1 Petrophysical characteristics at surface pressure
conditions
The physical properties along the core are presented in Fig. 3. The
density of the core samples varied relatively little throughout the
uppermost part of the core (upper schist series). Most values range
between 2700 kg m−3 and 2800 kg m−3 , with a small decreasing
trend with increasing depth. In the lower schist series the density
continues to decrease towards the pegmatite granites which exhibit
the lowest density of ∼2636 kg m−3 . Most samples throughout the
core exhibited minor porosity (<1 per cent). The overall magnetic
susceptibility distribution along the core indicates mostly paramagnetic behaviour (κ < 500 μSI) with a small ferromagnetic component due to small amounts of unevenly distributed magnetic fraction.
The NRM was generally weak throughout the core.
The interval from 1314 m to 1515 m is exceptional. This interval consists of rocks from the ophiolitic complex and shows large
variations in all physical properties (e.g. ρ from 2514 kg m−3 to
3158 kg m−3 ; κ from ∼0 to 6 × 10−2 SI).
While the rest of the petrophysical properties were nearly constant
throughout the core (especially in the upper schist series) the VP0
exhibits large variation and a slight decrease with increasing depth
(Fig. 5). The VP0 in the upper schist series varied between 3273
and 6174 m s−1 with an average VP0 of 5169 m s−1 . The ophiolitic
complex exhibited higher values of VP0 (≤7350 m s−1 ; average
5508 m s−1 ). The average VP0 decreased from 5169 m s−1 (the
upper schist unit) to 4512 m s−1 in the lower schist unit. Pegmatites
at the bottom of the core exhibit a higher average VP0 (5120 m s−1 )
than the samples from the lower schist series but these values are
still lower than the values of the rocks from the upper schist series.
Based on the petrophysical data (Fig. 4; Tables 1 and 2) the
different lithologies can be distinguished from each other. The
metasediments, such as mica schists, chlorite-sericite schists and
biotite gneisses exhibit overlapping petrophysical properties. Samples from these lithologies all show small variations in density and
porosity with average values of ρ = 2739 kg m−3 (φ = 0.5 per cent),
2726 kg m−3 (0.4 per cent) and 2792 kg m−3 (0.6 per cent), respectively. The magnetic susceptibility of those samples is 70–800 μSI
(one specimen showed κ = 3170 μSI). The mica schists exhibited larger variations in remanence than the biotite gneisses. While
some of the physical properties of biotite gneisses and chloritesericite schists are similar, the density, and especially the VP0 in the
biotite gneisses differs from those recorded for the chlorite-sericite
schists. Hence these two lithologies can be distinguished from each
other by means of density and VP0 (Figs 4d and e).
The black schists exhibit higher density (2852 kg m−3 ) than the
mica schists as well as wider variation in magnetic susceptibilities and NRM. The serpentinites, together with pegmatite granites
and quartzites, exhibit on average lower density (2621 kg m−3 ,
2636 kg m−3 and 2641 kg m−3 , respectively) than the metasediments. However, while the mica schists, biotite gneisses, quartzites
and pegmatites show para- or diamagnetic behaviour some serpentinites show presence of unevenly distributed ferromagnetic minerals (and thus higher κ and NRM), setting them apart from the other
lithologies (Fig. 4). The serpentinites were also more porous than
the other samples.
Pegmatite granites appear as distinct clusters of petrophysical
properties compared to the other lithologies in Figs 4a–d.
5.2 Ultrasonic seismic velocities at in situ-like pressure
conditions
Both the VP0 results obtained in laboratory conditions and the VP
and VS values with a pressure corresponding to what they face in
situ, varied considerably (Fig. 5). The average VP and VS values for
the upper schist series are 5509 m s−1 and 3132 m s−1 , respectively.
The variation seen in the VP /VS ratio, ranging from 1.63 to 2.05,
derived mainly from variation in VS rather than in VP . The average
VP /VS ratio was 1.76 for this series.
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Geophysical Journal International Laboratory seismic data of the Outokumpu core
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–1
–
Figure 3. Petrophysical properties (density, porosity, magnetic susceptibility and NRM) of OKU deep drill core as a function of depth. Blue arrow illustrates
possible reduction trend in density and increase in porosity.
There was no depth dependence observed in VS , however VP
increased slightly with depth (Fig. 5). The Poisson ratio (ν) of
these samples is 0.2–0.34 (0.26 on average). The seismic impedance
(ZP ), calculated from the measured density and velocity data, of the
upper schist series is on average 15.1 × 106 kg m−2 s−1 (14.9 ×
106 kg m−2 s−1 in the upper part) whereas the corresponding Young’s
modulus is 67.9 GPa. The samples from the ophiolitic complex show
on average 5622 m s−1 for VP and 3076 m s−1 for VS with a VP /VS
ratio of 1.85, ν = 0.29, ZP = 15.50 × 106 kg m−2 s−1 and E =
67.1 GPa, respectively.
Our studies indicate that the mica schists (Table 2), the majority
of the measured samples, show 5501 m s−1 for VP and 3124 m s−1
for VS with a VP /VS of 1.77 and a ν of 0.26 (ZP = 15.1 ×
106 kg m−2 s−1 and E = 67.5 GPa). The chlorite-sericite schist
exhibit higher VP and VS (also ZP and E) than the mica schists. For
chlorite-sericite schist VP /VS = 1.63 and ν = 0.20 was obtained,
whereas the biotite gneiss exhibit lower VP and VS (VP /VS = 1.83;
ν = 0.29). The serpentinites differ most from the other samples
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Geophysical Journal International featuring the highest VP /VS (2.06) and Poisson’s ratio (0.35), as well
as the lowest impedance (1.38 × 106 kg m−2 s−1 ).
6 DISCUSSION
The upper schist series, together with the lower schist series, is
mineralogically the most uniform rock unit of the drilled section
(Kukkonen et al. 2007a). This is supported also by the basic petrophysical data (e.g. magnetic properties: indicating mainly paramagnetic behaviour and no stable remanence; Fig. 3), which show
no distinct differences in any petrophysical properties throughout
both units. The seismic velocity data, in contrast, indicates large
velocity variations within these series, both in the laboratory conditions (VP0 ) and when applying in situ–like pressure conditions (VP )
to the samples. The average velocities in mica schists (Table 2) in
the current study are 5501 m s−1 for VP and 3124 m s−1 for VS
for in-situ pressure conditions while chlorite-sericite schist exhibit
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Tiiu Elbra et al.
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–
Figure 4. Correlation between (a) density and porosity; (b) density and magnetic susceptibility; (c) susceptibility and NRM (including Q-ratio, representing
the ratio of remanent to induced magnetization, as the line parameter); (d) density and VP0 and (e) porosity and VP0 of various lithologies.
higher VP and VS (5636 and 3464 m s−1 , respectively) and biotite
gneiss yield lower velocities (VP 5367 m s−1 , VS 2947 m s−1 ). The
lithological differences partly explain the intrinsic velocity pattern
(Fig. 5), but not the fluctuating values within the same lithology.
The seismic velocities in the samples can be determined by many
factors: rock forming mineral composition, fabric and structural
anisotropies, density, porosity, fracturing, dislocations, as well as
by the applied pressure and temperature (e.g. Kern 1993; Vajdova
et al. 1999; Kern et al. 2001, 2009; Kuusisto et al. 2006). Kern
et al. (2009) reported on the modal mineralogy and chemistry of
the lithologies present in the OKU core. These investigations concluded that the modal composition along with the shape preferred
orientation of the biotite grains partially contribute to the variation
seen in the elastic wave velocities. In general, the P-wave velocity
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Geophysical Journal International Laboratory seismic data of the Outokumpu core
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Table 1. Basic petrophysical properties of various lithologies. Displayed are average values of each lithology.
Lithology
Mica schist
Biotite gneiss
Chlorite-sericite schist
Black schist
Diopside-tremolite skarn
Serpentinite
Serpentinite-tremolite-(olivine) rock
Quartz
Pegmatite granite
n
ρ (kg m−3 )
φ (per cent)
κ (μSI)
NRM (mAm−1 )
269 (154/105)
26 (20/5)
6 (4/1)
7 (5/4)
2
12 (7/10)
3
2
32 (32/8)
2739
2792
2726
2852
3034
2621
2765
2641
2636
0.53
0.64
0.43
0.85
0.31
3.09
0.71
0.21
0.45
302
403
280
417
128
13790
234
−22
28
1.99
0.94
0.27
538.45
2.42
1080.48
2.47
0.05
0.27
n, Number of samples, the values in brackets indicate the number of samples used for porosity/NRM if differed from
n; ρ, wet density; φ, porosity; κ, magnetic susceptibility; NRM, intensity of the natural remanent magnetization.
Table 2. Elastic properties of various lithologies. Displayed are average values of each lithology.
Lithology
Mica schist
Biotite gneiss
Chlorite-sericite schist
Black schist
Diopside-tremolite skarn
Serpentinite
Serpentinite-tremolite-(olivine) rock
Quartz
Pegmatite granite
n
VP0 (m s−1 )
VP (m s−1 )
VS (m s−1 )
VP /Vs
ν
ZP E6 kg m−2 s
E GPa
98 (1218)
4 (107)
1 (18)
1 (36)
1 (35)
1 (86)
(15)
(5)
(256)
4919
4577
5763
5577
6267
5054
6266
5830
5396
5501
5367
5636
5431
6395
5039
-
3124
2947
3464
3134
3645
2448
-
1.77
1.83
1.63
1.73
1.75
2.06
-
0.26
0.29
0.20
0.25
0.26
0.35
-
1.51
1.47
1.54
1.56
1.71
1.38
-
67.5
62.9
78.7
66.8
90.4
44.3
-
n, Number of samples, in brackets is indicated the number of samples used in VP0 measurements; VP0 , P-wave velocity measured in ambient
pressure using velocity instrument in GSF; VP , P-wave velocity measured under estimated crustal in-situ pressure conditions; VS , S-wave
velocity measured under in-situ pressure conditions; ν, Poisson’s ratio [calculated using formula ν = ((VP /VS )2 –2)/(2((VP /VS )2 –1))]; ZP ,
seismic impedance [ZP = ρVP ]; E, Young’s modulus [E = (μ(3λ+2μ)/(μ+λ), μ = ρVS 2 , λ = ρVP 2 –2μ].
of unloaded mica schists and biotite gneisses is lower than what was
recorded for in-situ like pressure conditions (Table 2). The density
and porosity along the drilled section (Fig. 3) is nearly constant.
The density as a function of velocity (Fig. 4d) shows that the local density alone cannot explain the velocity variations. However,
despite the low porosity (<1 per cent in most samples) the results
suggest that the velocities are very sensitive to microfracturing. For
instance VP0 decreases dramatically with increasing porosity (Fig.
4e) in all rock types. Microfracturing could therefore explain the
velocity differences along the drilled crustal section. This explanation is supported by the increase in VP with increasing depth (and,
thus, pressure; Fig. 5) compared to the decreasing trend in VP0 as a
function of depth seen in the unloaded samples.
The microcracks in the OKU drilled crustal section can occur
for several reasons: (i) microcracks in in-situ rocks, (ii) drilling
induced microcracks or (iii) fracturing due to the stress release
in rocks during the extraction of the core. A slight increase in
porosity with depth within similar lithologies (Fig. 3) suggests that
the fractures are not drilling induced. The decompaction of the
samples (e.g. Kern et al. 2001; Kern et al. 2009) during core retrieval
may, however, explain the velocity (in ambient pressure conditions)
decrease with depth. This idea is supported by the observation that
the recompaction during the loading process closes the microcracks,
which reduces the porosity and increases the velocity (Fig. 5). This
observation is common in seismic velocity studies of the upper
crust (e.g. Kuusisto et al. 2006 and the references therein; Kern et al.
2009). In fact, oriented microcracks contribute largely to the seismic
anisotropies and to the reduced seismic velocities in the Kola super
deep core. Such oriented microcracks allow discriminating between
core samples and surface rocks (Kern et al. 2001).
C
2010 The Authors, GJI, 184, 405–415
C 2010 RAS
Geophysical Journal International Several studies determining the seismic velocities, as well as magnetic properties, have been conducted on surface rocks. In a Finnish
survey Kuusisto et al. (2006) compared the velocities adopted from
wide-angle velocity models with modified laboratory sonic velocities measured in surface rocks (by Christensen & Mooney 1995;
Christensen 1996). Unfortunately, their focus was on the lower crust
leaving the uppermost 5 km uncovered. Our studies focused on the
2.5 km crustal section, especially on the first 1.5 km. A VP less than
5600 km s−1 , a VP /VS ratio of 1.68–1.72 (Hyvönen et al. 2007), a ν
of 0.13–0.37 and an E of 37–95 GPa (Hakala et al. 2007) have been
reported for Finnish mica schists and biotite gneisses. In addition to
the mineralogical- and microfracture-induced velocity variations,
Kern et al. (2009) reported significant foliation-related velocity
anisotropy, especially at low pressures, for most of the OKU lithologies. For example, the values for VP and VS as measured down-core
(normal to the foliation plane) under 35 MPa (representing 1.3 km
depth) using dry samples, reveal a VP of 4915 m s−1 and a VS of
3088 m s−1 for the upper schist section which are weaker than their
bulk averages. This hints to structural (foliation) anisotropy (Kern
et al. 2009). A VP of 4160 m s−1 and a VS of 2275 m s−1 for serpentinites have also been reported by Kern et al. (2009). All our velocity
measurements were conducted normal to the foliation plane (z-axis)
and are thus minimum values. In foliated rocks the directional dependence (anisotropy) of the wave velocities at low pressures is
caused by microcracks, in addition to crystallographic (CPO) and
shape preferred orientation (SPO) of major minerals (muscovite
and biotite) and is strongly related to the structural frame (foliation,
lineation; Kern et al. 2008). The velocities recorded for the upper
schist series as well as for the serpentinites agree with surface data,
even though they are slightly higher (especially VP ) than the data
412
Tiiu Elbra et al.
–1
–1
–1
Figure 5. Seismic velocities and their ratio, impedances, Poisson’s ratio and Young’s modulus of the upper schist series and ophiolite complex under
corresponding in situ pressures (diamonds). VP0 refers to velocity measurements in laboratory pressure conditions (bar chart). Blue arrows indicate the increase
of VP and decrease of VP0 with depth.
reported by Kern et al. (2009). This can be due to the water saturation
of our samples, which increases VP , leaving VS as unaffected (e.g.
Benenson et al. 2002; Popp & Kern 1994; Schön et al. 2004). The
diopside-tremolite skarns exhibit, however, lower velocities than
reported by Kern et al. (2009) perhaps due to mineralogical differences or sample conditions, or due to the presence of more cracks.
The VP /VS , ν and E of mica schists and serpentinites, the latter featuring the highest VP /VS (2.06) and Poisson’s ratio (0.35) as well as
the lowest impedance (1.38 × 106 kg m−2 s−1 ) and Young’s modulus
(44.3 GPa) of the OKU drilled section, are consistent with those reported by Christensen (1996); Grasselli & Egger (2003); Wang & Ji
(2009). Furthermore, the magnetic properties indicate also the consistency to the published surface data. The susceptibilities of mica
schists are generally low (0–500 μSI) while the ophiolitic rocks
are reported to exhibit higher values (0–100000 μSI) depending on
magnetic mineralogy and its concentration, especially on the abundance of ferromagnetic pyrrhotite (Airo & Loukola-Ruskeeniemi
2004; Ruotoistenmäki & Tervo 2006). These data agree with our
results (κ < 500 μSI for mica schists and biotite gneisses and
κ < 6 × 10−2 SI for serpentinites), thus support the interpretation
that the magnetic anomalies occur due to the occurence of ophiolites
(Ruotoistenmäki & Tervo 2006).
In 2001–2005 the GSF carried out wide-angle seismic profiles
along four main transects with a total length of 2104 km. The effort was done within the framework of the FIRE (Kukkonen et al.
2006). The goal of the effort was to improve the understanding
of the crustal structures of the central part of the Fennoscandian
Shield. These FIRE sections featured a large variety of reflectors.
In addition to the uppermost surface reflectors (0 ∼ 200 m), one of
the most distinct upper crustal reflectors was observed in the Outokumpu area at 1.3–1.5 km depth (Fig. 6; OKU-1; Kukkonen et al.
2006). As previously described, the OKU deep drill hole, located
about 400 m W from the OKU-1 line (Heikkinen et al. 2007), cuts
through an ophiolitic complex, which resides at the same depth as
the observed reflectors in the Outokumpu area. To lithologically
and geophysically explain the reflectors, we have calculated the
reflection coefficients of various pairs of lithological units down
the drill core using the velocities measured under crustal conditions as described above. According to Warner (1960) and Kern
et al. (2009) reflection coefficients of RC > ±0.1 are required to
cause the strong seismic reflections. The reflection coefficients obtained in the current study (Table 3; Fig. 6) show that contacts between the diopside-tremolite skarns and mica schists (RC = 0.126;
contact at 1325.4 m), diopside-tremolite skarns and serpentinites
C 2010 The Authors, GJI, 184, 405–415
C 2010 RAS
Geophysical Journal International Laboratory seismic data of the Outokumpu core
413
Figure 6. Left: Finnish seismic reflection profile (OKU-1; marked as yellow line on FIRE 3 transect in Fig. 1a) with approximate location of OKU deep drill
core (adopted from Heikkinen et al. 2007). Right: reflection coefficients obtained in this study. Red diamonds denote the RC calculated from lithology means
(presented also in Table 3).
Table 3. Reflection coefficients for each pair of two lithological units.
Lithology
[1] Mica schist
[2] Biotite gneiss
[3] Chlorite-sericite schist
[4] Black schist
[5] Diopside-tremolite skarn
[6] Serpentinite
[7] Pegmatite granite
[1]
0.003
−0.01
−0.014
–
–
0.023
[2]
[3]
−0.003
0.01
0.012
–
–
–
–
0.02
–
–
–
–
[4]
0.014
–
–
−0.112
–
–
[5]
[6]
[7]
0.126
–
–
0.112
−
–
–
−0.08
−0.19
−0.023
−0.02
–
–
–
–
0.19
–
–
Coefficients are calculated from lithology means using formula RC = (ρ 2 VP2 -ρ 1 VP1 )/(ρ 2 VP2 +ρ 1 VP1 );
numbers in [] refer to rock type; ‘–’ lithological contact not met in the drill core (based on Västi 2005).
(RC = ±0.19; within 1332–1493.15 m) as well as between diopsidetremolite skarns and black schists (RC = ±0.112; 1451.5–1514.3
m) can produce the strong reflections as observed in the FIRE
profile.
7 C O N C LU S I O N S
The results indicate that various lithologies can be distinguished by
their petrophysical data. Weak depth dependence in seismic P-wave
velocity, density and porosity is observed. The seismic velocity
data shows large variations, whereas the density and the porosity
are nearly constant along the core. The magnetic susceptibilities and
remanence distinguish between the magnetically weak schist series
rocks and occasionally ferromagnetic rocks in the ophiolite series.
The laboratory measurements indicate that the seismic velocity
is significantly affected by microfracturing, which is the main cause
of the observed velocity variations. These variations are reduced
when applying upper crustal pressures. Nevertheless, the velocities
are still minimum values due to fabric and structural anisotropies.
The measured velocities provide estimates for the reflection coefficients for major lithological boundaries. These data indicate that
the strong and distinct reflections visible in the FIRE wide-angle
reflection data within the 1325.4–1514.3 m interval are caused by
contacts of diopside-tremolite skarn into serpentinites, into black
schist or into mica schist.
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2010 The Authors, GJI, 184, 405–415
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Geophysical Journal International AC K N OW L E D G M E N T S
We thank the staff of the petrophysical laboratory of GSF in Espoo
(especially Mrs. Satu Vuoriainen) for preparing the samples and for
allowing us to use the VP0 -velocity instrument. We also acknowledge
Dr. Ilmo Kukkonen for his help and Mr. Robert Klein for language
checking. The authors thank the Outokumpu Oyj and the K.H.
Renlund Foundations for financial support to construct the new
ultrasonic device for measuring the P-wave and S-wave velocities
under in situ like conditions. The reviewers, Prof. H. Kern, Dr.
D. Brown and N.N. are thanked for constructive reviews, which
significantly improved the paper.
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